Advances in photovoltaic cells remain important for terrestrial and non-terrestrial applications. In the non-terrestrial environment of outer space, as well as terrestrial applications, photovoltaic cells offer a valuable means for providing power generation by converting the abundant resource of the sun's energy to electrical power.
Irrespective of the application, and as with any energy generation system, efforts continue to increase the output and/or efficiency of PV cells. With respect to output, multiple cells or layers having differing energy bandgaps have been stacked so that each cell or layer can absorb a different part of the wide energy distribution of photons found in sunlight. Stacked arrangements have been provided in monolithic structures on a single substrate, or on multiple substrates.
In the multiple cell device, known as multijunction solar cells, multijunction photovoltaic cells, or multijunction cells, semiconductor materials are typically lattice-matched to form the solar cells within the multiple cell device, known as subcells within the multijunction solar cell. Each subcell typically contains at least one collecting p-n (or n-p) junction. A multijunction solar cell with 2 subcells is typically called a 2-junction cell; one with 3 subcells is called a 3-junction cell, etc. so that cells with n subcells are called n-junction cells, where n is an integer.
Additionally, the subcells within a multijunction cell are often interconnected in series by tunnel junctions between subcells, that act as low resistance contacts that typically are not photoactive. In contrast, the collecting junctions in each subcell typically are photoactive. The term photoactive means that a given photoactive layer, structure, subcell, etc. within a solar cell contributes to the output current and/or voltage of the overall solar cell, in response to light incident on the solar cell. As described earlier in the text, the numbering of 2-junction (2J), 3-junction (3J), and, in general, n-junction (nJ) solar cells is determined by the number of subcells, or collecting junctions, not including tunnel junctions.
The collecting junction of a photovoltaic solar cell or subcell typically consists of a p-n junction between a layer of one doping type (either p-type or n-type), typically called an emitter layer, and another layer of the opposite doping type, typically called a base layer. The junction may also consist of a p-i-n junction in which an intrinsic semiconductor layer with little or no extrinsic dopant concentration is placed between the emitter layer of one doping type and the base layer of the opposite doping type. Typically, the emitter layer is considered to be the layer that is closer to the primary light source for the solar cell than the base, and the base layer is considered to be the layer that is farther from the primary light source than the emitter. Typically, the front surface of a solar cell or solar cell component is considered to be the surface closer to the primary light source for the solar cell, and the back surface of a solar cell or solar cell component is considered to be the surface farther from the light source. However, there can be exceptions to this typical terminology, for instance when both back surface and front surface illumination are incident on the solar cell.
Both the collecting junctions and the tunnel junctions can be of the homojunction or heterojuction type. When solar energy is absorbed by a subcell, minority carriers (i.e. electrons and holes) are generated in the conduction and valence bands of the semiconductor materials adjacent the collecting junction. A voltage is thereby created across the junction and a large portion of the photogenerated current can be collected at a finite voltage to generate electrical power. As the photons in the solar spectrum pass to the next junction that typically has been optimized for a lower energy range, additional photons in this lower energy range can be converted into a useful current. With a greater number of junctions, greater conversion efficiency and increased output voltage and electrical power can be obtained.
With multijunction cells, efficiency is limited by the requirement of low resistance interfaces between the individual cells to enable the generated current to flow from one cell to the next. In a monolithic structure, tunnel junctions have been used as described earlier in the text to minimize the blockage of current flow. In a multiple wafer structure, front and back metalization grids with low coverage fraction and transparent conductors have been used for low resistance connectivity.
Another limitation to the multiple cell PV device, or multijunction cell, is that current output in each subcell must be the same for optimum efficiency in the series-connected configuration. In addition, there has been a practical limit on the number of subcells (also referred to as collecting junctions or subcell junctions) employed, since the benefit of each successive subcell becomes smaller as the number of subcells increases, and each subcell has certain parasitic losses associated with it in practice, tending to counteract the greater efficiency that comes from dividing the incident spectrum into smaller energy ranges with a greater number of subcells.
The concern over efficiency in PV cells has created interest in optimizing 1-junction solar cells such as silicon (Si) cells and gallium arsenide (GaAs) cells, and in developing higher-efficiency multijunction cells such as conventional 3-junction GaInP/GaInAs/Ge solar cells, which employ a gallium indium phosphide (GaInP) top subcell, also called cell 1 or C1, a gallium indium arsenide (GaInAs) subcell in the cell 2 (C2) position, and a germanium (Ge) subcell formed by a Ge wafer growth substrate in the cell 3 (C3) position. In this 3-junction solar cell, cell 3 is also the bottom subcell. Thus the subcells are numbered in the order in which the incident light passes through the multijunction cell. The material name associated with a solar cell or subcell is typically the material that is the dominant photoabsorber in the cell, which is typically the base of the solar cell.
The structures described above have relatively high current densities, which can present problems for current matching subcells that are poor current producers. The comparatively high current densities and low voltages of these cells with 1 to 3 subcells result in greater relative power loss due to series resistance. Further, subcell base thicknesses can be comparatively large, and some subcells have little excess photogenerated current density, both of which impair radiation resistance. Limited excess photogenerated current density in low bandgap subcells can also impair the fill factor of the overall multijunction solar cell that they are in.